Various additives are added to lubricating oils such as engine oils, drive system oils, and metal working fluids for the purpose of enhancing the performances thereof. Various additives are available, but additives containing metals are generally used in many cases. If a metal is contained in a lubricating oil, for example, there arise problems such that the metal is released to the environment to cause various adverse effects, and a process for discharging the lubricating oil becomes complicated. Therefore, extensive studies have been made to develop a lubricating oil with as low a metal content as possible.
One of the most desirably replaced metal-containing additives is zinc dithiophosphate. Zinc dithiophosphate is an additive containing phosphorus and zinc atoms, but has antioxidant ability and abrasion resistance, and hence the compound is used in various lubricating oils. The reason why zinc dithiophosphate is so disliked is because the zinc dithiophosphate has various adverse effects as described above. However, among those, when the zinc dithiophosphate is added to automotive engine oil, phosphorus atoms released together with exhaust gas adhere to automotive exhaust gas catalysts, resulting in lowering the catalyst activity. If the catalyst activity is lowered, harmful substances such as nitrogen oxide and sulfur oxide contained in the exhaust gas may be released to the atmosphere without being decomposed by the exhaust gas catalyst.
However, if the amount of zinc dithiophosphate is reduced to a certain level or less, antioxidant ability is significantly degraded. Therefore, it was impossible to reduce the amount of zinc dithiophosphate below a certain level. Also, if the amount of an antioxidant such as a phenol-based antioxidant or an amine-based antioxidant is in place of the reduced amount zinc dithiophosphate, sludge in the lubricating oil increases as the lubricating oil deteriorates. Therefore, it was impossible to deal with the problem by increasing the amount of such antioxidants sdded.
Accordingly, antioxidant compositions having various compositions have been developed. For example, Patent Document 1 discloses an antioxidant system containing (A) a secondary diarylamine, (B) at least one sulfurized hindered phenol, and (C) an oil-soluble sulfur-containing molybdenum compound. Meanwhile, Patent Document 2 discloses a composition which contains a lubricating oil and at least a first antioxidant and a second antioxidant, and further the first antioxidant is a secondary diarylamine and the second antioxidant is 2,2,4-trialkyl-1,2-dihydroquinoline or a polymer thereof.
Patent Document 3 discloses a lubricating composition containing, as essential components, an oil-soluble molybdenum compound obtained by reacting one or two or more compounds selected from molybdenum trioxide, molybdic acid, and alkali salts thereof with a reducing agent, and then reacting the resultant compound with a monophosphate or a diphosphate, and a sulfur-containing compound.    Patent Document 1: JP 2001-089782 A    Patent Document 2: JP 2002-531632 A    Patent Document 3: JP 62-39696 A